Granular additive for electrorefining of steel

ABSTRACT

Slag recovered from a previous electroslag remelting of a metal or similar thereto is dried and ground into a fine powder and then mixed with a metal to be added to an electroslag melt, e.g., of steel, in order to alloy it or deoxidize it. The mixture is formed into granules having a density closer to that of the slag than to that of the metallic component from which the granules were formed. These granules can even be lighter than the slag in the melt, so that when added they float thereon. Thus, when the granules are charged onto the melt they spend a relatively long time in contact with the relatively hot slag so that their nonmetallic component is absorbed into the slag while their metallic component is slowly released and thereby afforded sufficient time to heat up and react with the slag or to melt completely before passing into the body of molten metal udner the slag.

United States Patent [1 1 Oct. 30, 1973 Metz [ GRANULAR ADDITIVE FOR ELECTROREFINING OF STEEL [75] Inventor: Paul Metz, Luxembourg,

Luxembourg [73 Assignee: ARBED Acierle Reunies de Burbach-Eich-Dudelange, de la Liberte, Luxembourg, Luxembourg [22] Filed: July 12, 1971 [21] Appl. No.: 161,957

[30] Foreign Application Priority Data July 10, 1970 Luxembourg 61306 Sept. 25, 1970 Luxembourg 61766 [52] U.S. Cl. '75/10 R, 75/93, 75/129, 148/26, 164/52 [51] Int. Cl. C22d 7/06, C22b 9/10, C22c 33/00 [58] Field of Search 75/10, 11, 12, 53, 75/58, 30, 43, 93, 96, 129, 134; 148/26; 164/52 [56] References Cited UNITED STATES PATENTS 3,627,592 12/1971 Schmidt 148/26 3,344,839 10/1967 Sunnen 164/52 3,258,328 6/1966 Goss 75/53 3,469,968 9/1969 Snow 75/10 2,076,712 4/1937 Fink 75/96 3,244,510 4/1966 Goss 75/129 3,446,614 5/1969 Johnson 75/129 3,234,608 2/1966 Peras 75/10 FOREIGN PATENTS OR APPLICATIONS 979,583 l/l965 Great Britain 75/10 OTHER PUBLICATIONS Duckworth & Hoyle: Electroslag Refining, 50 (1969).

Primary Examiner-L. Dewayne Rutledge Assistant ExaminerPeter D. Rosenberg Attorney-Karl F. Ross [57] ABSTRACT Slag recovered from a previous electroslag remelting of a metal or similar thereto is dried and ground into a fine powder and then mixed with a metal to be added to an electroslag melt, e.g., of steel, in order to alloy it or deoxidize it. The mixture is formed into granules having a density closer to that of the slag than to that of the metallic component from which the granules were formed. These granules can even be lighter than the slag in the melt, so that when added they float thereon. Thus, when the granules are charged onto the melt they spend a relatively long time in contact with the relatively hot slag so that their nonmetallic component is absorbed into the slag while their metallic component is slowly released and thereby afforded sufficient time to heat up and react with the slag or to melt completely before passing into the body of molten metal udner the slag.

7 Claims, No Drawings a 1 GRANULAR ADDITIVE FOR ELECTROREFINING OF STEEL FIELD OF THE INVENTION The'present invention relates to the electroslag melting of a metal and, more particularly, to an additive for an electroslag refining or alloying melt, e.g., to supply alloying ingredients or refining agents such as deoxidizers. The invention also relates to a method of making the additive and to a method of remelting metals using same.

BACKGROUND OF THE INVENTION In electroslag melting a solid charge of a metal introduced into a mold or a crucible is melted in a layer of molten slag. This slag which is a conductor of electricity when molten is held at the required temperature at which the metal melts by the electric current passing from one or more consumable or non-consumable electrodes, the lower end of which enters into the slag, through the molten-conductive slag to the base of the mold or crucible.

Heat being is withdrawn from the mold by cooling the pool of molten metal beneath the slag layer to subject the melt to a progressive oriented solidification.

The process has been used heretofore simply for the remelting of solid metal, e.g., ingots, scrap, ingots and scrap together for the alloying of the melt with one or more alloying metals (e.g.,chromium, nickel, molybdenum, manganese, tungsten or vanadium in the case of steel), and/or for the refining or treatment of a melt (e.g., for the deoxidation of a steel bath by the addition of aluminum thereto). In the electroslag process, the slag consists generally of fluorides, oxides and mixtures thereof and is, in effect, a flux promoting heating andmelting-refining of the metal.

As noted, it is known to add certain products to a melt in an electroslag mold in order to combine with the metal to form an alloy, or to react with the slag. Such additives usually consist of a relatively dense metal. I I

In the case of'alloy formation the metal is usually just charged onto the melt continuously as a divided powder which, due to its density, sinks through the slag and enters into the melt. This powder has a composition different from that of the melt and from that of the electrodes in order to impart a particular composition to the body of molten metal.

A great disadvantage of such a method is that the powder tends to agglomerate either on the cold walls of the ingot mold when it is carried as lumps into the bath when the level of the slag layer rises, or between the electrodes where it creates a short circuit (being more conductive than the slag). This latter disadvanta'ge is particularly great in cases where the refilling factor, i.e., the ratio of the cross-sectional area of the electrodes to the cross-sectional area of the crucible is large. In this case the free space between electrodes and between the electrodes and the wall of the vessel is meager. In addition to and in part due to these difficulties the finished ingots often exhibit marked segregation and inhomogeneity of the distribution of the added substance throughout their cross-section.

Attempts to use a less finely divided metal as the additive have also been unsuccessful. In this case the separate metallic granules pass rapidly through the relatively hot layer of slag without having time to melt, and

enter into the somewhat cooler body of molten metal in a solid condition. In some'cases'aetual pieces of the alloying metal can be found in the finished ingot, clearly an undesirable condition.

In order to produce ingots'having a very low oxygen content it is also known to introduce reducing metals into the melt. Aluminum for example is added to the slag to form aluminum oxide therein and simultaneously increase the deoxidizing capacity of the slag. Such metals are added in the form of pieces of wire, bands, strip or the like to the melt. Apparatus for the charging of such an additive must be provided in addition to the apparatus for charging the necessary alloying metal or metals.

OBJECTS OF THE INVENTION It is therefore an object of the invention to provide an improved method of refining a metal.

Another object is the provision of an improved additive useful in the alloying of steel.

-Yet another object is to provide an additive which can combine the functions of deoxidizing or otherwise react with slag with that of alloying.

A further object is to provide an improved alloying additive which mixes well with and melts completely in the body of molten bath or slag.

SPECIFIC DESCRIPTION These above objects are attained according to the present invention which provides in the continuous or discontinuous electroslag melting of a metal, the admixing of an additive formed of porous granules themselves constituted by a finely divider powder of the metal to be added mixed with a lighter nonmetal. The density (specific gravity) of these porous granules is thus lower than that of the additive metal, some relatively heavy metal, and close to that of the nonmetallic base or substrate which has a relatively low density close to that of the slag. Such granules do not simply sink through the slag, but remain therein for-a'time and guarantee the sustained release of the additive over this period. The very hot slag thereby melts away the nonmetal base and combines therewith, thereby liberating the heavy metal either to react with the slag or to pass through the slag in a fully melted condition and mix with the metal melt. Due to the relatively long time the metal will pass in the hotter slag layer it will be completely melted so that the finished ingot will be almost perfectly integrated.

I have found, more particularly, that the use of composite granules having a density equal to or slightly greater than that of the slag entering into the composition, i.e., between 3.2 g/cm and 4.5 g/cm, leads to results which are as good as those which can be obtained with slags of a density intermediate that of the metallic additive products and that of the remelting slag used to form the mixture, i.e., between 4.5 g/cm and 6.5

which prevents them fromagglomerating and making a segregated ingot.

Thus, it is necessary that the granules have an overall density or specific gravity which is less than that of its metallic component or components. Generally such densities lie between 3.2 g/cm and 6.5 g/cm".

Metallic components usable in the granules can be, for example, titanium, molybdenum, vanadium, tungsten, manganese, niobium, chromium, cobalt, nickel, or copper. These metals are usually used in the form of ferroalloys or base-metal alloys with several components.

It is also possible to include in the granules metals serving to restore the reducing power of the slag for the production of low-oxygen steel. Aluminum or reducing ferroalloys are used for instance. Of course when no alloying is required only the reducing metals need be included in the granules along with, of course, the relatively light mineralic base component.

The base is formed of a mixture or compound corresponding to that of the slag in the melt, even as far as 1 relative proportions of the components are concerned in order to avoid any disadvantage change in the melt after addition. Fluorides, oxides, or mixtures thereof have been found useful. The electrical conductivity, viscosity, fusion temperature, and basicity of the nonmetallic component of the additive granules should be calculated so as not to upset those characteristics of the existing slag. When it is found necessary to mix a binder with the component it has been found advantageous to use a binder chosen preferably amongst the hydrocarbons which are completely destroyed in the subsequent fusion of the granules.

In the case of a nonmetallic base component which is hygroscopic or hydrous, the water must be removed prior to use. This can be done by melting it for several minutes in an are or induction furnace, as is the case with remelting slags before use. It is therefore possible to employ a slag from a previous or different melt as mineral base for the granules, preferably a slag which was only used once. The dehumidified base is ground up to the desired mesh size and mixed fully with the also dried and finely divided metallic component. A binder is added if necessary and porous granules are formed by bricketting or pelletizing. Moisture must be eliminated or avoided at every step since it is difficult to eliminate hydrogen from the melt once the hydrogen has been absorbed.

With the above method it is possible to impart to the finished granules or pellets a by-weight composition of 75 percent of the metal and 25 percent of the nonmetal, the overall density of the granules being much closer to that of the nonmetallic composition than to that of the metal. With a smaller percentage of metal in the granules it is even possible to attain overall densities which are less than the density of the lighter nonmetallic component and, therefore, lighter than the slag on the melt. It is not necessary that the granules be physically strong; they need only be sufficiently resistant to withstand handling prior to use.

The additive according to the present invention is so light it rests for a relatively long time on or in the slag layer. This gives the metallic component sufficient time to either react with this slag or to fuse completely. The resultant tiny drops of metal in the slag have a relatively large surface area in comparison to their mass so that they heat up well. The composition of the slag is, of

course, not affected since thelightweight material of the granules has precisely the same composition.

SPECIFIC EXAMPLE In order to make lightweight porous granules containing 40 percent by weight of chromium the necessary quantity of ferrochrome having a chromium content of 65 percent by weight was ground up to a granule size of 0.4 mm to 0.8 mm. Slag consisting of 60 percent by weight CaF 20 percent by weight of A1 0 and 20 percent by weight of CaO was also reduced to this mesh size. The ferrochrome and the ground-up slag were mixed completely and then compacted in a press into pellets which were then broken up into granules of an average diameter between 2 mm and 4 mm and an overall density of 3.50 g/cm.

These granules were charged continuously into the crucible of an electroslag refining apparatus according to a proportion calculated with methods well known in the art. Thus these 40 percent-chrome granules were used to make steel ingots having a 2.5 percent by weight chrome content by remelting steel ingots free of chrome. Samples taken from several different places on a plurality of different ingots thus made had a chromium content varying between 2.42 percent and 2.57 percent by weight, which shows remarkable integration.

I claim:

1. A method of smelting a metal comprising the steps of: I

forming a slag-covered melt of a metal by electroslag heating; producing a granular additive by compacting a mixture of a finely divided metal with a finely divided nonmetallic composition to produce a body having a density substantially between 3.2 g/cm and 6.5 g/cm and less than that of the metal of said melt, and granulating said body to form said additive;

charging particles of said granular additive onto said melt; and

heating said additive in said slag to fuse said nonmetallic composition and thereby free said metal of said additive.

2. The method defined in claim 1 wherein said nonmetallic composition consists essentially of fluorides, oxides, or mixtures thereof.

3. The method defined in claim 1 wherein said metal is reactable with the slag on said melt.

4. The method defined in claim 1 wherein said metal passes through said slag and into said melt after fusion.

5. An additive for treatment of a slag-covered metal melt comprising granules of a compacted mixture of finely divided metallic component and a nonmetallic component, said granules having an overall density substantially less than that of said metallic component between 3.2 g/cm and 6.5 g/cm and corresponding generally to that of the slag on said melt.

6. A method of smelting a metal comprising the steps of:

forming a slag-covered melt;

. recovering slag from a previous but similar melt and converting same into a dry nonmetallic powder; mixing said powder with a metallic powder;

forming granules from the powder mixture with a density substantially lower than that of said metallic powder;

charging said granules onto said melt; and'heating said granules with said slag to fuse said nonmetallic powder and free said metallic powder.

7. The method defined in claim 6 wherein said granules are formed with a density corresponding generally to that of the slag on said melt.

* i i i 

2. The method defined in claim 1 wherein said non-metallic composition consists essentially of fluorides, oxides, or mixtures thereof.
 3. The method defined in claim 1 wherein said metal is reactable with the slag on said melt.
 4. The method defined in claim 1 wherein said metal passes through said slag and into said melt after fusion.
 5. An additive for treatment of a slag-covered metal melt comprising granules of a compacted mixture of finely divided metallic component and a nonmetallic component, said granules having an overall density substantially less than that of said metallic component between 3.2 g/cm3 and 6.5 g/cm3 and corresponding generally to that of the slag on said melt.
 6. A method of smelting a metal comprising the steps of: forming a slag-covered melt; recovering slag from a previous but similar melt and converting same into a dry nonmetallic powder; mixing said powder with a metallic powder; forming granules from the powder mixture with a density substantially lower than that of said metallic powder; charging said granules onto said melt; and heating said granules with said slag to fuse said nonmetallic powder and free said metallic powder.
 7. The method defined in claim 6 wherein said granules are formed with a density corresponding generally to that of the slag on said melt. 